The present invention relates generally to metal foils and methods of making the same. More particularly, this invention pertains to the fabrication of very thin, unbacked metal foils. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Thin metal foils are of use in various well known applications, for example, as elements of electrical devices, and as windows, filters, and targets for radiation and atomic particle beams. Additionally, thin foils are of current research interest as plasma generators. In the latter application, a large electrical charge is discharged through a foil to vaporize it and produce a plasma. Shaped foils, i.e. foils which have a curviplanar configuration, are of particular interest in this application because the plasma momentarily assumes the shape of the preexisting foil. For example, tubular foils have been of particular interest because they are useful in studies of magnetic implosion of plasmas. For various reasons, it is desirable that such foils be free of seams or creases, and be as thin as can possibly be made.
Most metal foils are manufactured by a mechanical process involving extrusion and pressing of metal sheets. The lower thickness limit obtainable with such a process is on the order of 25 .mu.m, which is too thick for the applications referred to above.
Where thinner metal foils are required, a vacuum vapor deposition process is ordinarily employed. In such a process a metal is vaporized and subsequently condensed onto a solid substrate to form a thin foil on the substrate. In one previously known variation of this method, a metal is vaporized and deposited onto a glass plate which is coated with a water-soluble mold release agent such as cesium iodide. When the metallized plate is subsequently immersed in a water bath, dissolution of the underlying mold release agent causes the foil to separate from the glass and to become freely suspended in the water. Foils as thin as 0.07 .mu.m have reportedly been made by this process. The foil thus made is very delicate and easily torn, and can ordinarily be handled only while still suspended in the water bath. Even with such careful handling, however, such foils can usually be successfully transferred only to a flat, substantially solid surface. It is most difficult and usually impossible to drape or stretch such a foil over an open framework or lattice-type structure so as to obtain an unbacked foil sheet which is of substantial size and which is free of surface defects. For the same reason, it has not been possible to form shaped foils by this method, as for example by wrapping the foil around a cylindrical lattice to form a foil tube.
It has also been previously known to form thin backed metal foils by vacuum vapor deposition of a metal onto a plastic film. Such a procedure is disclosed, for example, by F.W. Kindel et al., in "Thin Foil Fabrication," Review of Scientific Instruments, 50, 1550 (1979). The advantage of this variation of the vapor deposition process is that the resulting foil/plastic laminate is relatively sturdy and can be handled as required. It has not been previously possible, however, to separate the foil produced by such a procedure from the underlying plastic film. Accordingly, the film remains with the foil as an integral backing element in whatever application the foil is employed. In some applications, however, the plastic film backing is unnecessary and even undesirable because it is electrically nonconductive, and because the foil/plastic laminate is substantially thicker and heavier than the foil sheet alone.
Accordingly, it is an object and purpose of the present invention to provide a method of making a thin, unbacked metal foil.
It is also an object of the invention to provide a method of making a shaped metal foil sheet having a desired three-dimensional configuration, and which is free of seams, creases, and other discontinuities. In this regard, it is a particular object of the invention to provide a method of making a thin-walled, seamless foil tube.
It is another object of the invention to provide a method of making a thin, shaped metal foil of substantially uniform thickness.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.